For a virus to cause a major nationwide or worldwide health problem, there has to be a 'perfect storm' of different factors coming together:

The virus must be different enough from other viruses which have recently infected humans that very few people have been infected with something similar. That means very few people will be immune to it, increasing the 'reservoir' of people who can get infected and pass it on. Every year the flu virus changes - that's why if you're eligible for a flu vaccine, you'll be invited to have one every year. However, in most years the change in the virus is fairly minor, meaning that quite a lot of people who have had flu before are immune to it. This minor change is called 'antigenic drift'. The Asian flu pandemic of 1957 and the Hong Kong flu pandemic of 1968 were both caused by this major change in an existing virus - called 'antigen shift' .

The virus must cause serious illness in a high proportion of healthy people infected. There was huge anxiety about the H1N1 'swine flu' virus, which first emerged in Mexico in 2009, because it was both new and able to spread easily between people. However, the infection it caused turned out to be relatively mild in most people.

The virus must be able to spread easily between people. The 'bird flu' H5N1 was first identified in 2003, but since then it has only caused about 500 human infections, and almost all of these were in people who had been in close contact with birds.

The latest new coronavirus was first identified in patients from Qatar and Saudi Arabia in September last year. So far, twelve cases have been confirmed and five have died. Of the four cases in the UK, the first was in a patient flown in from Qatar for treatment. The second was in a patient who had recently been in the Middle East and Pakistan. The third was in a relative of the second, who had a medical condition which could make him more vulnerable to severe infection. Both these people have a severe form of infection affecting the lungs, needing treatment in a hospital intensive therapy unit (ITU) and in some of the other people infected it has caused kidney failure too. The latest is another member of the same family - they didn't have any medical conditions and their infection, so far, seems very mild.

All this may sound very worrying, but there are many reasons for us not to panic. Firstly, there have been only 12 cases worldwide since the virus was identified in September - if it was easily passed from person to person, there would have been far more. Secondly, all the people in close contact to people infected have been tested, and no others have been found to have the infection. Thirdly, coronaviruses cannot survive for longer than about 24 hours outside the human body and are killed by most strong detergents. This makes it different from viruses like rotavirus which spread like wildfire. They can survive for long periods in water or surfaces, allowing them to pass between people even if they aren't in direct contact. Of course, one death from a new virus is one death too many - but at least it isn't likely there will be many more.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.